chore: add link checking in CI

[huitseeker]

This adds a link-checking action run on a weekly basis (Mondays) using lychee:

The check can also be triggered manually using the Github CLI or the 'Actions' tab interface for contributors of the repo.

This does not act on PRs but rather publishes / updates an issue on the repo with the link check report.

Demo run:
https://github.com/huitseeker/sui/runs/6282514162?check_suite_focus=true

Demo resulting issue:
huitseeker#4

We should also include a configuration file ./lychee.toml with possible excludes and other details, see documentation. This is a first stab, though.

[Clay-Mysten]

Adding here since we never created an issue for this change:

Me: May I ask to be made the default assignee on these reports? FWIW, I
Francois: You can do that yourself by a PR against the GH action.
here's the doc:
https://github.com/peter-evans/create-issue-from-file#action-inputs
here's the line that needs changing:

sui/.github/workflows/links_checker.yml

Lines 56 to 62 in 9eb55f8

	with:
	title: Link checker report
	content-filepath: ${{ env.LYCHEE_OUT }}
	issue-number: ${{ steps.last_issue.outputs.issue_number }}
	labels: |
	report
	automated issue

Done:
#1961
MystenLabs/narwhal#242